1. Technical Field
The present invention relates to a technology for controlling behavior of a light-emitting element, such as an organic light-emitting diode element (hereinafter, referred to as ‘OLED’ element) or the like.
2. Related Art
In general, electro-optical devices, in which a light-emitting element, such as an OLED element or the like, is used, have been suggested as display devices of various electronic apparatuses. This kind of electro-optical device has a structure in which a plurality of pixel circuits, each of which has a light-emitting element, are disposed in a matrix. Each of the pixel circuits controls a current supplied to the light-emitting element.
FIG. 28 is a circuit diagram illustrating a structure of one pixel circuit in an electro-optical device according to the related art (for example, see ‘2001FPD Technology Outlook’, Electronic Journal, p 749 to 750). As
shown in FIG. 28, a pixel circuit P0 includes a p-channel-type transistor Tdr (hereinafter, referred to as ‘driving transistor’) that is interposed between a power supply line 31 and a ground line 32, and a light-emitting element 17. Each of the power supply line 31 and the ground line 32 is commonly connected to the plurality of pixel circuits P0 that are disposed in a matrix. A high-side potential VH of a power supply, which is generated by a power supply circuit (not shown), is supplied to each pixel circuit P0 through the power supply line 31, and a low-side potential VL, which is generated by the power supply circuit, is supplied to each pixel circuit P0 through the ground line 32.
As shown in FIG. 28, a gate terminal of the driving transistor Tdr is connected to a first terminal of a capacitor element C0 and a drain terminal of an n-channel-type transistor Ts1 (hereinafter, referred to as ‘selecting transistor’). A second terminal of the capacitor element C0 is connected to the power supply line 31. In the meantime, the selecting transistor Ts1 is a switching element that controls an electrically conductive state and an electrically non-conductive state between a data line 13 and the first terminal of the capacitor element C0 in accordance with a level of a scanning signal Ssel. The data line 13 is supplied with a potential Vdata hereinafter, referred to as ‘data potential’) which corresponds to a gray-scale level designated for each pixel circuit P0.
In this configuration, if the selecting transistor Ts1 shifts an off state to an on state by the scanning signal Ssel, the corresponding data potential Vdata is supplied to a gate terminal of the driving transistor Tdr, and then held in the capacitor element C0. In addition, a current Ie1, which flows through the ground line 32 from the power supply line 31 via the driving transistor Tdr and the light-emitting element 17, is controlled in accordance with a voltage herd in the capacitor element C0. Accordingly, the light-emitting element 17 emits light with a gray-scale level (luminance) according to the data potential Vdata.
In the meantime, due to a resistance being generated in the power supply line 31, in the potential VH supplied to each pixel circuit P0, the voltage drop according to the location of the corresponding pixel circuit P0 (specifically, length of a path from the power supply circuit to the pixel circuit P0) is generated. Accordingly, the potentials VH supplied to the respective pixel circuits P0 are different from each other while depending on the corresponding locations of the respective pixel circuits P. In addition, there is a problem in that a gray-scale level of a light-emitting element 17 of each pixel circuit PD may vary due to the difference between the potentials VH. This problem will be described in detail below.
In FIG. 28, if a driving transistor Tdr operates in a saturation region, a current supplied to the light-emitting element 17 is represented by the following Equation (A1)Ie1=(½)β(Vgs−Vth)2  [Equation A1]
In this case, in Equation (A1), ‘β’ indicates a gain coefficient of the driving transistor Tdr, ‘Vgs’ indicates a voltage between a gate terminal and a source terminal of the driving transistor Tdr, and ‘Vth’ indicates a threshold voltage of the driving transistor Tdr. A voltage Vgs when the selecting transistor Ts1 is turned off becomes the difference between the potential VH of the power supply line 31 and the data potential Vdata (Vgs=VH−Vdata). Therefore, Equation (A1) is changed to the following Equation (A2).Ie1=(½)β(VH−Vdata−Vth)2  [Equation A2]
As such, in the structure shown in FIG. 28, the current Ie1 that actually flows through the light-emitting element 17 (and a gray-scale level according to the current Ie1) depends on the potential VH of the power supply line 31. Therefore, it becomes difficult for the plurality of light-emitting elements 17 to emit light in accordance with a common gray-scale level. As a result, even if the same data potential Vdata is supplied to the pixel circuits P0, the potentials VH supplied to the respective pixel circuits P0 may be different from each other due to the voltage drop in the power supply line 31. Due to this, the current Ie1 that actually flows through the respective light-emitting elements 17 varies, so that the luminance is different for every light-emitting element 17.